'match' vs 'if defined', different behavior

When a symbol is defined as nothing (which is entirely valid and useful in certain cases), this can be detected by the 'match' statement, but not by 'if defined'.

When defining symbolic constants, 'if defined' works only if a value is given to the constant; if it is defined as nothing, an error is reported (error: invalid expression). This is because, after the preprocessor does its job by replacing the symbolic constant with nothing, the instruction 'if defined MYLABEL' becomes 'if defined', which is an error.

In the sample below, 'define MYLABEL' and 'MYLABEL equ' both cause errors for 'if defined'. The problem is solved by assigning some value to the label, such as 'define MYLABEL 1' or 'MYLABEL equ 1':

Code:

; Compile the code below as is, and then with the 'define' line commented out
define MYLABEL  ; Causes an error for 'if defined' test below
                ; When changed to 'define MYLABEL 1', it works with all cases below
                ; Similar results with 'MYLABEL equ' vs 'MYLABEL equ 1'
                                
; Test if defined with 'match' statement...
; One of these always works, whether 'define' line is commented out or not
; But when MYLABEL is given a value, it never thinks it is defined!
match ,MYLABEL {
  display 'match indicates label is defined',13,10
}
match x,MYLABEL {
  display 'match indicates label is NOT defined',13,10
}

; Test if defined with 'if defined' statement...
; The test for 'defined' does not work when defined as nothing
if defined MYLABEL
  display 'label is defined',13,10
else
  display 'label is NOT defined',13,10
end if

I hope this info helps others avoid unnecessary time figuring out how to eliminate 'if defined' errors.

EDIT:
Use the following match statement to detect when label is defined as 1:

Code:

match =1,MYLABEL {
  display 'match indicates label is defined',13,10
}